Deciding settings of over current relay for Inverse Time Characteristics.

Using this graph for standard inverse characteristics and grading margin of 200 ms as above; we will decide settings of the relay for the example power system. These settings can be decided considering three different co-ordination (Setting PSM and TMS for selective tripping) methods. Limitations of co-ordination method A and B are overcome through co-ordination method C. 

Co-ordination Method-A: As shown here under we will set the relay R1, R2 and R3 for operation time of 600 ms, 400 ms and 200 ms respectively considering fault at T/F HV Side.

                       Co-ordination Method-B: As shown here under we will set the relay R1, R2 and R3 for operation time of 900 ms, 700 ms and 500 ms respectively considering fault at T/F LV Side.

                      Co-ordination Method-C: As shown here under we will set relay R3 for operation time of 500 ms considering fault at 33kV Bus. Adopt HS instantaneous current setting for relay R3 as 2000 A and proper time gradation for relay R2 and R1 accordingly.